One of the things that keeps me awake at night is – ironically – something I used to take to help me sleep.

For a decade I consumed what's known as a first-generation antihistamine just before bedtime. Anyone who has taken these for allergies or as a sleeping aid will know that they can rapidly make you feel drowsy.

For me, and many friends, it's been a necessary component in the midlife insomnia toolkit. But, increasingly, studies are linking long-term use to a higher risk of dementia in later life. Have I inadvertently stacked up serious future problems in the desperate quest for uninterrupted kip?

So what's the problem? These drugs have been around for more than 80 years and some are specifically sold as temporary sleeping aids.

Brands such as Piriton, Phenergan, Nytol (not the herbal one) and Boots Sleepeaze (these last two are sleep aids and it's one of the uses for Phenergan) are effective for allergies, motion sickness, nausea and insomnia, working by blocking histamine – a chemical released by the immune system to protect the body – which triggers allergic symptoms.

But that's not all. 'They also block the release of a brain chemical called acetylcholine,' explains Dr Ahmad Khundakar, head of integrated science and associate professor of pharmacy at Teesside University in Middlesbrough. 'This chemical is vital for brain function, regulating processes such as thinking and attention.'

This is where my concern lies. Long-term use of drugs that interfere with acetylcholine has been linked to cognitive decline and an increased risk of dementia in older adults.

A US study of 3,500 women and men aged over 65 found those using these drugs were more likely to develop dementia, and the risk increased with the cumulative dose.

This is why now, aged 53, I'm increasingly worried about my overly enthusiastic antihistamine consumption. It's worth saying that this is entirely on me, and not the medication's fault. Lead a middle-aged woman to a sleeping solution, and you can bet your bottom dollar she'll take it, whatever the 'short-term use only' instruction warnings. Or perhaps I'm just especially idiotic.

The trouble is they've been part of my life for decades. As a small child I had catastrophic hay fever. My parents always kept two brown glass bottles in the fridge, both containing brightly coloured liquids – the antihistamines that allowed me to function during the summer.

As an adult, I turned to more modern medication, such as loratadine, but suffered from insomnia in my late 30s (it was probably perimenopause, now I think about it).

One evening, remembering that it historically made me drowsy, I tried a swig of the allergy syrup I'd bought for the children and, lo and behold, I sank into a swift, deep sleep.

This became a habit over the next couple of years, and I gradually ended up taking it in tablet form most nights year-round (as I say, very much not recommended).

It became a crutch and, as life became increasingly busy and stressful, sleep seemed elusive without the sweet oblivion I believed was being provided by one pill.

Now, with my memory sometimes patchy and often struggling to find the right word, I find myself panicking that there will be a price to pay.

I am not alone. My friend Camilla, also in her 50s, says she has the same fear. 'I've taken an antihistamine two or three times a week to help me sleep for years. I'm terrified I'm storing up problems for later.'

Are we right to be scared? There's no question that there's an association. But these medicines are best described as a 'modifiable risk'.

'I'd be cautious about saying these drugs cause dementia,' says Dr Khundakar. 'The underlying disease process is usually already there. But if someone is on the road to developing dementia, large amounts of these drugs can certainly worsen things and have a long‑term impact on cognition. Two of the most common forms of dementia – Alzheimer's disease and dementia with Lewy bodies – involve a major loss of acetylcholine activity in the brain.'

I'm ashamed to admit I have glossed over my use with a flippant, 'Oh!' (fake surprise) 'Do they cause drowsiness?'

Pharmacist Wendy Lee, Well Pharmacy's professional and governance manager, is a bit shocked by my admission. 'The antihistamine in Piriton, for example, is short-lasting, so you need to take a tablet every four to six hours. A pharmacist will probably ask why you need it and recommend an alternative if it's for daily allergy use.'

Thankfully, for those panicking about hay fever season (particularly bad this year, with the Met Office issuing a red alert last month due to 'very high' pollen levels), second-generation antihistamines do not block the release of acetylcholine – these are loratadine, cetirizine hydrochloride and fexofenadine (brand names include Clarityn, Piriteze and Allevia) – this last I swear by for my hay fever.

As for the antihistamines sold for sleep problems, such as Boots' Sleepeaze, Lee emphasises they should only be used infrequently and for no longer than recommended.

Sleep expert Dr Neil Stanley warns a lack of shuteye is probably more dangerous than antihistamine use for sleep, while chronic insomnia is another dementia risk factor.

He also says I was wasting my time, adding: 'The thing about these first-generation antihistamines is that they don't keep you asleep and the effect will only last for four days.'

But I took them for years, I protest. 'Placebo effect,' he says, rather abruptly. Oh dear.

In terms of dementia risk, Dr Khundakar points out that in most cases, the root cause is unknown, but thankfully there's a lot we can do.

'For the common, late-onset type of dementia, genetics are only part of the story,' he says.

'Generally speaking, anything that's good for your heart is good for your brain. So exercise is thought to help boost cognitive function. Poor diet and excess alcohol consumption are known risk factors – as is, obviously, smoking.'

Thankfully, I gave up my antihistamine use when I turned 50, three years ago, and simultaneously increased my exercise regime, improved my diet (immeasurably!), and I have recently been using a vagus nerve device called SONA which has helped with my sleep.

The fear of dementia is present for many of us, but mitigating any risks makes sense. And that includes ditching my unnecessary antihistamine habit.

Consult a medical professional before making any changes to medication